While many are chattering about high-speed rail these days, state transportation leaders are quietly planning to drop more than $15 billion into California's existing Amtrak train service -- including a big chunk here in the Valley.

Improvements for Amtrak's San Joaquin line are forecast in a draft of a new statewide rail plan that the California Department of Transportation is circulating for public comment through March 11.

The plan offers a vision of how California's system of freight and passenger trains will look in 2020. In addition to high-speed rail -- construction is planned to start this summer in Fresno -- there are improvements to tracks, stations and other features of Amtrak routes and commuter train lines in the Sacramento/Stockton area, the San Francisco Peninsula and Southern California.

With a high-speed line proposed to connect to commuter systems at either end of the state and span a gap across the Tehachapi Mountains between Bakersfield and Palmdale, the plan envisions "connecting the whole state together for the first time in many years with passenger rail," said Brent Ogden, a plan consultant.

The plan identifies more than $560 million in improvements to tracks, signal systems and stations for Amtrak's San Joaquin corridor within the next five years, and more than $1.7 billion in the corridor over the next 20 or more years.

The improvements are intended to increase the frequency and speed of Amtrak trains, improve passenger safety, boost ridership, reduce greenhouse gas emissions, connect to other transit systems and build new facilities for expanded passenger rail service.

In the San Joaquin Valley, the first section of high-speed rail from Merced to Bakersfield is proposed to be the backbone of a bullet-train system linking the Bay Area and Los Angeles.

But in the decade before high-speed trains start running, the plan anticipates that the high-speed tracks could be used by Amtrak's San Joaquin trains that now share tracks with BNSF Railway freight trains between Bakersfield and Stockton.

By 2020, as many as 11 daily Amtrak San Joaquin trains could be rolling on high-speed tracks at speeds up to 125 mph. Up to six additional Amtrak trains would continue sharing the BNSF freight tracks each day, stopping at Hanford, Corcoran, Wasco and Madera.

"It's going to depend on marketing and timing. But we are anticipating that we will continue to serve those communities," Bill Bronte, the rail division chief for Caltrans, said at a San Joaquin Rail Advisory Committee meeting Thursday in Fresno.

Bronte added that Caltrans expects to preserve Amtrak train service in the Valley even after high-speed 220-mph trains are proposed to begin operating between Merced and the San Fernando Valley.

He said that in addition to serving smaller Valley cities that won't have high-speed rail stations, Amtrak will remain a lower-priced option than the bullet trains for many travelers up and down the state. "We believe there will be demand because there will be a cost differential," he said.

Amtrak's trains would return to the BNSF tracks after the high-speed trains go into service.

The San Joaquin line from Bakersfield to Sacramento or Oakland, and two other Amtrak California routes -- the Pacific Surfliner between San Diego and San Luis Obispo, and the Capitol Corridor between Sacramento and Oakland -- are among the five busiest passenger train corridors in Amtrak's system.

In the final three months of 2012, the San Joaquin trains carried more than 393,000 riders. That was an increase of almost 11% from the same period in 2011 -- the largest ridership increase among all of Amtrak's short-distance corridors nationwide.